Studies in biological control and invasion biology rarely determine whether introduced plants may rapidly evolve in the area of introduction. Examining the evolution of plant chemical defenses after reassociation with a coevolved enemy is important not only to understand the dynamics of plant-herbivore interactions but also in predicting potential ultimate outcomes of classical weed control programs. A system in which chemical evolution may be examined involves the interaction between Conium maculatum, a Eurasian weed naturalized in North America that contains high concentrations of piperidine alkaloids (γ-coniceine, coniine, conhydrinone) acting as chemical defenses, and its monophagous European associate Agonopterix alstroemeriana. In the United States, C. maculatum was largely free from herbivory until approximately 30 years ago when it was reassociated via accidental introduction with A. alstroemeriana. At present C. maculatum and A. alstroemeriana are found in a continuum of re-association times and intensities. To determine the degree to which variation in alkaloid content and resistance to A. alstroemeriana were genetically based we conducted a common garden experiment involving plants from three locations in the U. S. (Illinois, Washington and New York) that had experienced 12, 20 and 32 years of reassociation with A. alstroemeriana respectively. We analyzed alkaloid concentrations and recorded the natural colonization of A. alstroemeriana on C. maculatum by counting the number of leaf rolls at the end of the season. Additionally, a bioassay with larvae from our laboratory colony raised on IL, NY and WA foliage was conducted to determine the effects of secondary chemistry on insect fitness.

Resum:

Total alkaloid production in C. maculatum was positively correlated with reassociation time between C. maculatum and A. alstroemeriana, with the longest historical association with A. alstroemeriana (NY) had highest alkaloid content; WA plants were intermediate, with the highest amount of variability among sites, and IL plants had lowest alkaloid concentrations. High variance among WA plants may be a result of transitioning from low alkaloid levels resulting from costs associated with investments in alkaloids in the absence of consistent herbivory, to a highly resistant state where variability has been stabilized due to selection by A. alstroemeriana. The number of leaf rolls present on C. maculatum was higher in IL plants compared to WA plants and larvae raised on foliage from IL experienced significantly lower mortality than larvae raised on foliage from WA. Prolonged reassociation between C. maculatum and its specialist moth may increase toxicity of this noxious weed in its introduced range. In a previous experiment we documented differences in alkaloid concentrations among plants from IL, WA and NY collected in situ (Castells et al. 2005 J. Chem Ecol. 31). Here we show that at least part of these differences, higher alkaloids in those regions with longer reassociation times (NY; WA ; IL), have a genetic basis. Plants with higher alkaloid concentrations experienced less damage by A. alstroemeriana. At the same time, a decrease in fitness was found when larvae were raised on high alkaloid plants. All of these results are suggestive of strong directional selection exerted by A. alstroemeriana on C. maculatum chemistry.